This invention is related to the electroless plating of silver onto graphite powder.
Bulk silver continues to increase in cost, prompting the search for alternatives for use in the fabrication, for example, of semiconductors and electronic devices. Silver-plated copper is one of the best alternatives due to its excellent initial conductivity. However, copper lacks oxidative stability, which limits its use in applications requiring high reliability at high temperature and high humidity conditions. Moreover, silver-plated copper itself is relatively expensive. Silver-plated glass or any other silver-plated filler with an insulator core suffer low conductive performance, and are poor substitutes for silver or silver-plated copper.
Silver-coated graphite is lower in cost than, and can deliver comparable initial conductivity to, bulk silver or silver-plated copper, without the oxidative stability problems associated with copper. Current processes for preparing silver-coated graphite, however, suffer from production difficulties.
The surface of graphite is inert and must be pretreated before it can be plated in an electroless process. However, graphite pretreatment methods involve at least one of the following steps: oxidation, heating, or wet chemical activation, followed by powder separation, washing and rinsing. All these procedures lead to problems for large-scale manufacture.
Oxidation is effective to introduce active sites on graphite surfaces for plating, but typical oxidants, such as nitric acid, sulfuric acid, or hydrogen peroxide, require special operation procedures due to their corrosive or explosive nature. In addition, powder separation, washing and rinsing generate hazardous waste.
Heating is another method to generate active surfaces on graphite. However, heating requires special equipment, there is a narrow temperature window for operation, and it is difficult to reproduce results.
Typical wet activation methods involve the use of tin or similar metal compounds, along with a sensitizer, such as, palladium chloride in aqueous condition. After sufficient mixing, the graphite powder must be separated from the activation bath using numerous filtration, washing and rinsing steps, taking time and creating hazardous waste.
The current invention circumvents these problems.